This document discusses stateful applications in Elixir. It provides examples of stateful applications like ExVenture, a MUD server, and Grapevine, a MUD community site. The document reviews why stateful applications may be useful in Elixir due to its ability to handle soft real-time capabilities, reduce database queries through caching, and make websites feel "alive". However, stateful applications also require more complex architectures using tools like OTP, come with cache management challenges, and are harder to deploy at scale. The presentation provides an overview of building blocks like GenServers, supervision trees and ETS for stateful applications in Elixir.

1. First title line
Second line here
Subtitle goes here — no fancy
color block kthough too fussy
smartly.
smartlogic.io
All Aboard
the Stateful Train
Going Beyond a Simple
Web App with Elixir
Eric Oestrich
smartlogic.io

2. Who am I?
Eric Oestrich
Wizard at SmartLogic

3. Who is
SmartLogic?
We build custom web
and mobile apps.
Based in Baltimore,
we've built over 150
apps since 2005.

4. ● What does a standard application look like?
● Why would you go stateful?
● What does a stateful application look like?
● Is stateful better?
● Let's go stateful!
Agenda

5. Standard Application

6. Standard Application
● Stateless HTTP
● Controllers go straight to the database
● Does not leverage OTP
● MVC, Rails-like

7. Standard Application Examples
● Wordpress
● Hubspot
● Wikipedia
● Shopify

8. Stateful Application

9. Stateful Application
● Stateless HTTP
● Websockets/TCP connections
● Heavy use of OTP
● "Background" processes

10. Stateful Application Examples
● Slack
● Discord
● Twitch

11. OTP and Processes
● Used to be Open Telecom Platform
● OTP is the standard library of Erlang
● GenServers, supervision trees, :ets, all the fun stuff
● Processes are the base building block of Erlang
concurrency

12. Supervision Trees & Processes

13. Why Stateful?

14. Why Stateful?
● We're on a platform that can handle it
● Local caches
● Speed, less network to the database
● Resilience, craft your processes to withstand anything
● Websites feel “alive”

15. Did I mention Erlang is great at this?
https://unsplash.com/photos/M5tzZtFCOfs

16. Let's review some stateful
applications

18. exventure.org
ExVenture: A MUD Server

19. What is a MUD?
● Multi-User Dungeon
● Online text adventure
● Connects with telnet

20. What is a text adventure?
This is an open field west of a white house, with a
boarded front door.
There is a small mailbox here.
A rubber mat saying 'Welcome to Zork!' lies by the
door.
>

21. MidMUD
The flagship ExVenture instance
midmud.com

22. Review of ExVenture
● A "living" world
● Everything is always on
● The database is there for startup
● Lots of inter-process communication

23. Demo
midmud.com

24. Could this be
stateless?

25. Not Really
● Poll based?
● Worker that updated the world?
● Gross

27. grapevine.haus
Grapevine: A MUD Community Site

28. Grapevine
● Steam for MUDs
● Cross game chat
● Web client

29. Review of Grapevine
● Close to a standard application
● State involved is websocket and telnet
● Caches

30. Demo
● Game listings, stats
● Cross game chat
● Web client

31. Could this be
stateless?

32. Possibly
● Chat applications can be poll based
● Live caches could hit the database
● Still gross

33. Is stateful better?

34. Do I want this?

35. Depends

36. Pros

37. Soft realtime
capabilities
Erlang is built for soft realtime

38. Website feels "alive"
Websockets can update your website without a refresh

39. Reduced
Database Queries
Less round trips to the database for faster response times

40. Cons

41. Requires heavy
usage of OTP
You must architect out how your application is laid out in
memory, beyond what Phoenix gives you.

42. Caches are Hard™
There are 2 hard problems in computer science: cache
invalidation, naming things, and off-by-1 errors.

43. Harder to Deploy
● Warm caches on boot
● Long lived connections will drop
○ How will you deal with this?
● Clustered applications need to handle a rolling restart

44. Rolling Restart

45. Here be Dragons
● Mailbox size
● Message passing copies
● Clustering woes

46. Let’s Go Stateful

47. GenServers
● The basic building block of OTP
● Handle messages out of the HTTP process
● Background tasks
● Any kind of state you want to hang on to

48. Supervision Trees
Watch over your GenServers

49. :ets
● Erlang Term Storage
● Very fast reads/writes
● Concurrent
● Downside: copies memory a lot
http://erlang.org/doc/man/ets.html

50. Put It All Together

51. Diagram of
ExVenture

52. Connecting
to the World

53. Players
Acting on
the World

54. Updating a
Skill from
the Admin

55. Using
a Skill

56. Grapevine
Chat

57. Grapevine
Web Telnet
Client

58. Wrap Up

59. Elixir gives you the
tools to build a
stateful application
with ease

60. Ram and CPUs are
cheap, use 'em

61. LiveView is an easy
way to start stateful

62. Finally, you might
not need it

63. GitHub
● oestrich on GitHub
● oestrich/ex_venture
● oestrich/grapevine

64. Titans of Text
titansoftext.com

65. Season 3 of Smart Software
podcast.smartlogic.io

66. Season 3 of Smart Software
podcast.smartlogic.io

67. Season 3 of Elixir Wizards
podcast.smartlogic.io

68. First title line
Second line here
Subtitle goes here — no fancy
color block kthough too fussy
smartly.
smartlogic.io
Thanks!
smartlogic.io

69. First title line
Second line here
Subtitle goes here — no fancy
color block kthough too fussy
smartly.
smartlogic.io
We build custom web
We’ve built 150+ applications since 2005.
Need a custom web/mobile app? We can help.
and mobile applications.
888-544-SMRT / contact@smartlogic.io